1. Field of the Invention
This invention relates to a thermal decomposition cell for producing molecular beams of an element from material gas.
2. Description of the Prior Art
A thermal decomposition cell is known. Such cell comprises a cell body, a crucible contained in the cell body, a heater for heating the crucible, a thermal decomposition baffle for producing a molecular beam from a material gas using heat from the crucible, and a gas supply tube for supplying a material gas of an element into the crucible through the bottom of the crucible. The material gas supplied to the crucible is subject to heat decomposition to produce a molecular beam used for epitaxial growth on a substrate placed on the top opening of the crucible. Generally, an epitaxial method using the molecular beam from a gas source is known as a thin-film-forming technique of the semiconductor materials. In this method, a material gas (for example, Arsine, Phosphine, etc.), which contributes crystal growth on the substrate is used. Generally, a thermal decomposition temperature of the material gas may be higher than the substrate growth temperature. In this case, the molecular beam is produced as follows:
The crucible and the thermal decomposition baffle of the thermal decomposition cell is heated to a temperature such that the material gas is subject to thermal decomposition. Then, the material gas is supplied into the crucible from the bottom of the crucible. The material gas is subject to thermal decomposition by contacting the thermal decomposition baffle, so that the molecular beam is produced. The molecular beam is supplied to a substrate on which crystal growth of the epitaxial layer is to be carried out.
In the conventional thermal decomposition cell, a thermal decomposition baffle made of pyrotitic Boron Nitride (PBN), metal tantalum, or the like is formed in a board. One or more boards of the thermal decomposition baffle are used in the crucible to heat the material gas. In such board type thermal decomposition baffle, the thermal decomposition baffle is formed to have a honeycomb structure to increase a contacting area of the material gas with the decomposition baffle to improve thermal decomposition efficiency of the material gas.
However, in the conventional thermal decomposition cell mentioned above, there is a limit on increasing its surface area because the thermal decomposition baffle is formed in a board. Therefore, it is difficult to increase the thermal decomposition efficiency of the material gas to a considerable extent. Accordingly, in the conventional thermal decomposition cell, in order to supply the molecular beam from a gas source to carry out epitaxial growth, it is necessary to introduce a large amount of material gas into the thermal decomposition cell. Therefore, there is a problem that an exhaust pump having a large capacity is required to introduce such a large amount of material gas.